This four-year K01 Mentored Research Scientist Development Award will provide me with advanced training and mentorship to become an independent investigator in implementation research, integrating methods from epidemiology, social science, and economics to increase engagement of young female sex workers (FSW) in HIV prevention services. My long-term goal is to become a leading investigator in implementation research that addresses health disparities in HIV prevention and care among hard-to-reach populations, with a focus on young key populations. The proposed work includes training and research that capitalize on the rich interdisciplinary research environment of RTI International. Training represents the foundation of this proposal and includes senior mentors and advisors with complementary expertise, didactic education, skills development, and mentored research so that at the completion of the award I will be well positioned for an independent research career. The training plan was developed in parallel with the mentored research plan to ensure that newly acquired knowledge and skills are immediately applied to the relevant aspects of the research project. The following objectives will be accomplished during this award: (1) obtain training and practical skills in implementation research methods, including methods to improve and evaluate implementation and delivery of combination health interventions in order to increase demand, uptake, retention and compliance to services by young key populations; (2) obtain training and practical skills in costing and cost-effectiveness analysis of combination health interventions; (3) become an independent investigator in implementation research by contributing to the field through scientific publications, being mentored by subject matter experts, and generating pilot data for a future R-series award. The goal of the proposed research component is to comprehensively evaluate a multicomponent HIV prevention project in Zimbabwe targeting young FSW <25 years of age, to determine methods of delivery that result in the highest uptake, retention, and compliance. Young FSW are a vulnerable population at greater risk for HIV acquisition than their older counterparts, yet thei engagement in HIV services remains low. Funding has been secured to implement an evidence-based, HIV prevention pilot project for young FSW, integrated within Zimbabwe's National Sex Worker Programme (NSWP) clinics. This research will comprehensively evaluate the pilot project at 8 NSWP clinics, benefitting from the infrastructure, resources, and wealth of programmatic data available from all 36 NSWP sites nationally. Specific aims are to: (1) determine barriers and facilitators to implementation and uptake of the HIV prevention pilot project; (2) implement and evaluate the pilot project using a randomized stepped wedge design; and (3) estimate the cost and cost-effectiveness of the different HIV prevention components. This study will provide quantitative and qualitative evidence to determine if a future trial to scae up and evaluate the intervention's effectiveness/cost-effectiveness is warranted.